My 1999 CLK320's battery goes flat after a few days being parked.
I thought the battery might have been on its way as it was more than four years old
So I bought a new battery and it's still doing the same.
Two days and it's dead as a door knob.

I called the electrician who checked it right through.
The battery is in excellent condition and there are no leaks in the car.

He reckons the alternator might have a faulty diode or two and thus is draining the power from the battery after being stopped for a day or so.

If the Auto 'leccy said there were no "leaks" in the car, I presume that he meant that there was no current draw from the battery with the engine off. That would be apart from any radio memory, body computer, etc, etc currents. There should definitely be no current drawn by the alternator.

If the alternator had faulty diodes causing the battery to go flat, then this should have been tested by the 'leccy and eliminated, or confirmed, as the cause.

If there were faulty circuit diodes then the charging current would be low.
To test this with your limited facilities, (hopefully you have a multimeter) check the battery voltage with the engine at idle and the lights, A/C, fan and rear demister on. You should have more than 12V for a good battery.

Increase the engine RPM to 2K or more.

The battery voltage should rise to 13.6v or higher. Normally it should not rise above 14.5V unless you are in an extremely cold area.

If not, then go to a different Auto 'leccy.

I hope this helps,
Tony

Life is a journey, with problems to solve, lessons to learn, but most of all, experiences to enjoy.

U must have a battery drain somewhere...disconnect the battery positive lead and connect an ammeter in series..there shud be less than 30Ma drain current (10-15ma is usual)...for an overview, read the excellent article on battery "parasitics" below:http://www.diagnosticnews.com/parasitic-battery-drains/

"The car alarm" These have caused me to pay for far too many batteries (now over 10). Now I just disconnect the terminals. A pain but your battery will last for 10 years. I generally don't park my cars in areas where they are likely to be stolen so the alarms are a pain in the ass. If you drive the car every day no problem, if you have a selection put the right size spanner in the car and just disconnect.

"The car alarm" These have caused me to pay for far too many batteries (now over 10). Now I just disconnect the terminals. A pain but your battery will last for 10 years. I generally don't park my cars in areas where they are likely to be stolen so the alarms are a pain in the ass. If you drive the car every day no problem, if you have a selection put the right size spanner in the car and just disconnect.

Hope this helps.

John

Just a side note to the alarm. Although we had alarms in our 3 w126 cars, we never bothered to activate them.
They were all fitted by the prev owners. The only precaution I take when leaving the car parked is to open the glove box.
This shows that there is nothing of value there.
To prove my point, we parked in a city car park many years ago. 12 cars that were parked alongside ours were broken into and ours was left untouched.
The only thing we did with the alarms was to rip them out when they started to drain the battery.